Hi Olivier,
my ideas go something like this:
* either a font with a strong maths association, like LaTeX's
"Computer Modern" font (a TTF port of that one is included in Ubuntu's
ttf-lyx package under the name of "cmr10")
* or, if serif fonts generally look too fuzzy at 24*24 (the icons are
always at this size, whether toolbars be "large" or "small", as far as
I can tell), a sans-serif font might be better. Among the free ones,
there are two that are used heavily within LibO
** Vegur – the branding font, might not be a good choice because of
glyph coverage issues and also because it's too closely tied to the
branding.
** Liberation Sans – used for some documentation and might eventually
replace Arial as the default document font, should have a good glyph
coverage.
Personally, I like FreeSans better than both of the aforementioned
options, because it is a plain Helvetica copy (unlike Liberation Sans
which is an altered descendent of Helvetica), however, it is not
really used much in the LibO context.
Hope that helps,
Astron